The present invention is generally directed to autoclaves and specifically to autoclaves having high rates of oxygen transfer to metal-containing solutions.
To oxidize sulfide sulfur and thereby permit solubilization and/or liberation of metals compounded with the sulfide sulfur, base metal ores and concentrates, and refractory gold ores and concentrates are commonly treated by pressure oxidation. Pressure oxidation is typically performed by passing a feed slurry of a metal-containing material through a sealed autoclave (operating at superatmospheric pressure) having multiple compartments. To provide for oxidation of the sulfide sulfur in the slurry, oxygen is typically fed continuously to the autoclave by means of a sparge tube located below the impeller. Commonly a large portion of the oxygen reacts with the sulfide sulfur, but there is a smaller significant portion that is vented from the autoclave and may be considered not effectively utilized.
In designing an autoclave, there are a number of considerations. By way of example, the autoclave should permit reaction of as much of the oxygen as possible with sulfide sulfur. If the oxygen is inefficiently reacted with the sulfide sulfur, the autoclave can have higher oxygen plant capital and operating costs. The autoclave should provide as short a residence time as possible for a given volume of slurry while realizing a high rate of recovery for the metal. Finally, the autoclave should vent inert gases that build up in the autoclave above the slurry to prevent rupturing of the autoclave from high pressure gas. Some oxygen gas is inevitably vented along with these inert gases. Other processes, which rely on efficient and effective gas/liquid transfer of oxygen and which are commonly carried out in autoclaves, include catalytic chemistry reactions, such as the conversion of ferrous to ferric ions, reoxidation of NO by oxygen, and cuprous amine conversion to cupric amine.
These and other design objectives are satisfied by the autoclave designs of the present invention.
In one embodiment, the autoclave includes a vessel for containing a feed slurry material, such as a metal sulfide-containing slurry, or a liquid comprising dissolved chemical compounds and one or more agitators with each agitator having one or more impellers attached to a rotatable shaft for agitating the feed slurry material. The shaft has a passage for a molecular oxygen-containing gas and an outlet in communication with the passage for dispersing the oxygen-containing gas in the slurry. In one configuration, the passage passes along the length of the rotatable shaft, and the outlet is located at or close to the tip of the impeller.
The autoclave can realize relatively high (molecular) oxygen transfer rates to the feed slurry material relative to conventional autoclaves through better oxygen gas dispersion in the feed slurry material. Commonly, the autoclave can yield an oxygen transfer rate of at least about 2 kg moles oxygen/cubic meter of slurry/hour. At such high oxygen transfer rates, a high rate of metal recovery can be realized in a relatively short residence time, and therefore lower capital and operating costs for the autoclave equipment can be realized relative to conventional pressure oxidation processes.
The autoclave is able to accomplish such relatively high oxygen transfer rates without the use of a sparge tube, though the sparge tube when used jointly with the agitator can produce even higher oxygen transfer rates.
To permit consumption of as much molecular oxygen as possible, the rotatable shaft of one agitator configuration has an inlet for the molecular oxygen containing gas located at an upper end of the shaft that is above the slurry surface yet is contained within the vessel. The inlet will provide a suction, drawing the atmosphere in the autoclave into the passage. After passing through the passage, the gas is dispersed into the feed slurry material. In this manner, the molecular oxygen is continuously recycled during pressure oxidation to provide a high rate of molecular oxygen utilization. By efficiently reacting the molecular oxygen, the autoclave can have lower (molecular) oxygen plant capital and operating costs than conventional autoclaves.
In another configuration, new molecular oxygen is supplied to the autoclave either directly through the rotatable shaft or through a separate conduit such as one having an outlet in close proximity to the agitator gas inlet or above the feed slurry material. In the latter case, the shaft includes the inlet at the upper end of the shaft to permit oxygen escaping from the agitated feed slurry material into the autoclave atmosphere and/or supplied directly to the atmosphere from an external source to be drawn into the shaft and thereby entrained in the agitated feed slurry material.
The agitator of the present invention can provide improved reaction rates in the upstream compartments of the autoclave. In conventional autoclaves, the initial compartments frequently operate at a temperature below the desired operating range (which is from about 180xc2x0 C. to about 220xc2x0 C.) because the exothermic conversion of sulfides to sulfates in the initial compartments is insufficiently complete to raise the temperature to the desired operating range. To raise the temperature to within this range, it is common to add steam (from a source external to the autoclave) to the initial compartments to raise the temperature of the slurry in the compartment and thereby increase the rate of conversion of sulfides to sulfates. Steam can be costly to add to the system. In contrast, in the autoclave of the present invention the agitator draws steam in the autoclave atmosphere through the shaft and into the slurry in the initial compartments, thereby providing a higher temperature in the slurry in these compartments and a concomitant higher reaction rate. In other words, the agitator increases the heat transfer from the discharge end of the autoclave (i.e., the downstream compartments) to the input end of the autoclave (i.e., the upstream compartments). Accordingly, the autoclave of the present invention can be less expensive to operate than conventional autoclaves that inject steam into the initial compartments.
In another embodiment, the autoclave includes a discharge control means for controllably removing the gas atmosphere from the sealed autoclave to prevent rupture of the autoclave from high pressure gases. The system typically includes:
(a) analyzing means (e.g., a gas analyzer) for analyzing a selected component (e.g., carbon dioxide and/or molecular oxygen) in the gas atmosphere inside the autoclave;
(b) an outlet for removing gas in the gas atmosphere from the autoclave interior;
(c) a controller (e.g., a computer) for receiving a signal from the gas analyzer and generating a control signal in response thereto; and
(c) a control means (e.g., a valve) for controlling the amount of gas removed in response to the control signal received from the controller. The control means vents the gas atmosphere when the amount of the component exceeds or falls below a threshold amount. In this manner, the autoclave can vent oxygen gas and other gases that build up in the autoclave above the slurry while maintaining the molecular oxygen gas in the autoclave as long as possible for consumption in the oxidation of sulfide sulfur.
In one embodiment, pressure oxidation using the autoclave is performed using the following steps:
(a) agitating a feed slurry material in the autoclave using an impeller, and
(b) during the agitating step (a), passing a molecular oxygen-containing gas through a rotatable shaft engaging the impeller and dispersing the gas radially outward from the shaft into the feed slurry material. In one autoclave configuration, the gas is passed through a blade of the impeller outwardly into the slurry.
In another embodiment of the invention, the agitator is used in conjunction with a sparge tube to provide a further increase in the (molecular) oxygen content of the feed slurry material. The sparge tube is preferably located in the vicinity of a lower impeller and more preferably is located beneath the lower impeller such that bubbles of the molecular oxygen-containing gas released by the sparge tube are dispersed in the vessel by the lower impeller. Alternatively, the sparge tube could be located directly below a gassing impeller which expels recycled gas into the slurry. In this configuration, the agitator may or may not include a second, mixing (e.g., nongassing, high shear, etc.) impeller.
The sparge tube can introduce fresh molecular oxygen (directly) into the slurry from a source external to the autoclave and/or recycle oxygen in the atmosphere of the autoclave into the slurry. Recycle using the sparge tube can be performed using an external gas recirculating pump or blower. In a particularly preferred configuration, the agitator (e.g., upper impeller) recycles molecular oxygen from inside of the autoclave into the slurry while the sparge tube introduces fresh molecular oxygen into the slurry (e.g., below the lower impeller) from a source external to the autoclave. In this configuration, the sparge gas is substantially free of recycled molecular oxygen and the sparge gas is introduced into the slurry at a different point than the recycled gas passing through the agitator. Typically, at least about 75% and more typically at least about 90% of the oxygen in the sparge gas is from the external source (e.g., has not yet been circulated through the slurry). Typically at least about 75% and more typically at least about 90% of the molecular oxygen in the recycled gas is from the autoclave atmosphere above the slurry (e.g., has been circulated through the slurry at least once). In this configuration, the two gas streams remain independent of one another until they are introduced into the slurry. In another configuration, the sparge tube recycles molecular oxygen from inside of the autoclave into the slurry while the agitator introduces fresh molecular oxygen into the slurry from a source external to the autoclave. As will be appreciated, the content or concentration of molecular oxygen (and nitrogen) in the fresh gas stream from a source external to the autoclave will typically be greater than a molecular oxygen (or nitrogen) content or concentration in the autoclave atmosphere (and therefore in the recycled gas stream). The concentration of water vapor and/or carbon dioxide in the fresh gas stream is typically less than the concentrations of the components in the recycled gas stream. As will be further appreciated, molecular oxygen is consumed in the autoclave during sulfide oxidation. Typically, the concentration of molecular oxygen in the gas stream from a source external to the autoclave is at least about 17.5% by volume and more typically at least about 85% by volume. The autoclave atmosphere typically contains no more than 50% by volume molecular oxygen and more typically contains from about 5 to about 20% by volume molecular oxygen. Typically, the concentrations of water vapor and carbon dioxide in the recycled gas each range from about 1 to about 90% by volume. In contrast, the fresh gas stream typically is substantially free of water vapor and carbon dioxide and more typically contains no more than about 0.5% by volume of either component.